1. Field of the Invention
The present invention is related to a clamping circuit and an electronic sensor which uses same. Particular utility for the present invention is found in the area of electromagnetic radiation sensing apparatus, although other utilities are contemplated for the present invention, including other types of sensing apparatus. Additionally, the clamping circuit of the present invention may be used in many different types of electronic devices, and thus, although it will be described in connection with use in sensing apparatus, it should not be viewed as being limited solely to use in this one type of apparatus.
2. Brief Description of Related Prior Art
FIG. 1 is a schematic diagram of a conventional electromagnetic radiation sensor 10. Sensor 10 includes an electronic transimpedance amplifier 12 that comprises an operational amplifier 16 having inverting and non-inverting inputs 18, 20, respectively, and output 28. The inverting input 18 of the operational amplifier is connected via a high impedance feedback path 22 to the output 24 of the amplifier 16, and is also conected, in parallel, to an electromagnetic sensitive photodiode 26 whose anode is connected to ground and cathode is connected to node 54. Typically, the photodiode 26 is made from silicon. Feedback path 22 consists of a resistor 30 (typically having a very large resistance value, e.g., on the order of 10.sup.9 ohms) connected between the output 28 and the inverting input 18 of the amplifier 16 via node 54. The non-inverting input 20 of the amplifier 16 is connected to ground potential.
In normal operation (i.e., when amplifier 16 is unsaturated), photodiode 26 responds to incident electromagnetic radiation within its frequency band of sensitivity (e.g., the optical frequency band) by generating a photocurrent I.sub.p that is related to the strength of said radiation. In response to the photocurrent I.sub.p generated by the photodiode 26, the operational amplifier 16 supplies feedback current I.sub.f through the feedback path 22. In this configuration, operational amplifier 16 is configured to keep the voltage levels of the inverting and non-inverting inputs 18, 20, respectively, equal to each other. Thus, because the non-inverting input of the amplifier 16 is held at ground potential, the operational amplifier 16 will seek to supply appropriate feedback current I.sub.f to negate the photocurrent I.sub.p generated by the photodiode 26 so as to prevent a potential difference from being generated across photodiode 26, and to thereby force the inverting input 18 of the operational amplifier to remain at ground potential. Thus, the feedback current I.sub.f generated by the amplifier 16 is equal in magnitude to the photocurrent I.sub.p generated by the photodiode 26, but opposite in direction thereto.
Thus, in normal operation of sensor 10, the magnitude of the output voltage V.sub.out of the operational amplifier 16 is equal to the magnitude of the photocurrent I.sub.p multiplied by the resistance of the resistor 30. Thus, during said normal operation, the magnitude of the output voltage increases in proportion to the magnitude of the photocurrent I.sub.p, and therefore, is indicative of the magnitude of electromagnetic radiation incident to diode 26.
Unfortunately, operational amplifier 16 has a limited output voltage range, and typically will saturate when the magnitude of the output voltage of the operational amplifier is at or near the magnitude of the power supply voltage being supplied to the operational amplifier (e.g., 15 V). When the operational amplifier is saturated, the magnitude of output current generated by the operational amplifier remains constant regardless of further increases in photocurrent generated by the photodiode. Thus, once sufficient photocurrent is generated to cause the operational amplifier to saturate, the operational amplifier becomes unable to generate additional feedback current to compensate for any further increases in photocurrent. This can cause the voltage level of the inverting input 18 of the operational amplifier to rise to a level equal to the uncompensated photocurrent multiplied by the impedance of the photodiode 26. Disadvantageously, this can de-stabilize the operation of the operational amplifier.
Also, since the photodiode 26 has an inherent parasitic capacitance, the potential difference generated across the photodiode 26 as a result of the aforesaid phenomena can cause the photodiode to become charged. The charge stored in the photodiode 26 is then discharged when the strength of the incident electromagnetic radiation falls below the level sufficient to cause saturation of the operational amplifier. Disadvantageously, discharge of the charge stored in the photodiode 26 artificially increases the photocurrent supplied by the photodiode above the amount that is truly indicative of the strength of incident electromagnetic radiation. This can cause the output voltage generated by the operational amplifier to not accurately indicate the strength of incident radiation striking the photodiode. Indeed, it has been found that after significant periods of charging, if the incident electromagnetic signal exciting the photodiode 26 is suddenly removed, the aforesaid discharging phenomena may keep the operational amplifier's output voltage at saturation level for as long as several seconds.
One conventional attempt to solve this problem is embodied in sensor 50 of FIG. 2. Sensor 50 includes all of the elements of sensor 10 of FIG. 1, but also includes a clamping diode 52 shunted across the feedback resistor 30 of the feedback path 22 and forward biased in the same direction as the flow of feeback current I.sub.f. In operation of sensor 50, when the output voltage of the operational amplifier 16 increases, the impedance of the clamping diode 52 decreases and thereby reduces the total resistance of the parallel combination of the feedback resistor 30 and diode 52. This permits an increased amount of total current to flow from output 28 to node 54 to cancel out photocurrent I.sub.p. Using this technique, it is possible to ensure sufficient current flow to node 54 from output 28 to negate any expected magnitude of photocurrent.
Unfortunately, however, the technique illustrated in FIG. 2 has at least one significant drawback. Diode 52 has its own inherent parasitic capacitance (typically at least 10 pF) which parasitic capacitance deleteriously effects the bandwidth and time constant of the sensor 50 of FIG. 2 when compared to the sensor 10 of FIG. 1. Indeed, it has been found that the inclusion of clamping diode 52 in sensor 50 can decrease the bandwidth and increase the time constant of sensor 50 by more than an order of magnitude when compared to the sensor 10 of FIG. 1.
An example of a conventional clamping circuit is described in U.S. Pat. No. 4,578,576 to Wheeler et at. Unfortunately, this patent suffers from the aforesaid and/or other disadvantages and drawbacks of the prior art discussed above.